Magnetic resonance imaging is an imaging technique that magnetically excites nuclear spins of an object placed in a static magnetic field with the use of an RF signal at the Larmor frequency for the spins, acquires an MR signal emanated in response to the excitation, and reconstructs an image from the MR signal.
In the field of MR imaging, fast imaging techniques have been researched actively in recent years. As one example, there has been known fast imaging which is generically called parallel MR imaging using a plurality of RF coils. This parallel MR imaging technique has also been referred to, from a historical viewpoint, as a multicoil fast imaging technique, PPA (Partially Parallel Acquisition) technique, or subencoding technique.
This parallel MR imaging technique has been executed in various modes. In the initial stage, there were provided modes proposed by papers, for instance, “Carlson J. W. and Minemura T., Image Time Reduction Through Multiple Receiver Coil Data Acquisition and Image Reconstruction, MRM 29:681-688, 1993” and “Ra J. B. and Rim C. Y., Fast Imaging Using Subencoding Data Sets From Multiple Detectors, MRM 30:142-145, 1993.”
There have been many techniques which have been upgraded from the initial ones. Among such upgraded techniques, included is a SMASH technique known by a paper “Sodikson D. K. and Manning W. J., Simultaneous Acquisition of Spatial Harmonics (SMASH): Fast Imaging with Radiofrequency Coil Arrays, MRM 38:591-603, 1997” and a SENSE technique known by a paper “Pruessman K. P., Weiger M., Scheidegger M. B., and Boesiger P., SENSE: Sensitivity Encoding for Fast MRI, MRM 42:952-962, 1999.”
These types of parallel MR imaging techniques require a fundamental imaging configuration, in which, what is called a multicoil composed of a plurality of RF coils (element coils) is used to receive echo signals from the RF element coils simultaneously and to produce image data from the echo signal received from each RF element coil. On condition that the plurality of RF coils provide simultaneous reception, the number of encoding times assigned to each RF coil is reduced down to a value obtained by dividing “a predetermined number of encoding times necessary for image reconstruction” by “the number of RF element coils.” This encoding technique leads to a smaller FOV for each image produced by an RF element coil, so that scan time is reduced, while the above encoding causes wrap-around or folding phenomena at the ends of each image.
FIG. 1 shows examples of images acquired by the parallel MR imaging technique under the condition of “the number of element coils=2.” Pictures (a) and (b) in FIG. 1 illustrate images acquired from different element coils, respectively.
In the parallel MR imaging technique, the fact that a plurality of RF coils each have different sensitivity distributions is used for post-processing, that is, unfolding processing for plural images. The unfolded plural images are processed into a final full-FOV (field of view) image. The unfolding processing makes use of spatial sensitivity maps for each of the RF coils (element coils). In this way, parallel MR imaging provides fast scanning (fast imaging) and provides a wide field-of-view final image, such as an image covering the whole abdominal region.
The sensitivity maps are required to be obtained before a main scan each MR imaging time. Practically, image data acquired by a pre-scan (preparation scan) is subjected to predetermined calculation so that the sensitivity data can be obtained. The thus-obtained sensitivity data then undergoes low-pass filtering or polynomial fitting processing, with the result that sensitivity maps for unfolding processing can be provided. In this calculation, at least either an interpolation method or an extrapolation method may be usable.
However, a region where echo signal sources are abundantly present provides large amounts of sensitivity information (i.e., echo signals) about each RF element coil, but a region where there are few echo signal sources, such as a region outside an object to be examined and the pulmonary region of the object, provides only a small amount of sensitivity information about each RF element coil. Thus, it is difficult to apply the foregoing low-pass filtering and polynomial fitting processing to an image acquired from a region where the density of echo data (original data) sources to be acquired is coarse. In addition, if it is desired to adopt the extrapolation method, the spatial range in an acquired image to which the extrapolation method is applicable is smaller, thus frequently a difficult situation is faced where sensitivity distribution data cannot be expanded with precision.
Further, when applying low-pass filtering or polynomial fitting processing to an acquired image, a peripheral portion of the image frequently shows abnormal diverging data, thus making it difficult to obtain a smooth and stable sensitivity map.
As a result, the unfolding processing should be performed with the use of a sensitivity map which is lower in both precision and smoothness, resulting in artifacts that still remain in the full-FOV synthesized image finally obtained. The image has a deteriorated quality and is low in a depiction performance.